1. Field of the Invention
The present invention relates to gas sensors and the manufacturing method thereof to determine freshness and/or putridity for vegetables or fruits by providing high sensitivity and high selectivity to low levels of gases, such as ethylene, ethanol, mercaptans, and amines, released from vegetables or fruits.
2. Related Art of the Invention
The freshness of foods or drinks is determined subjectively through vision, taste and sense of throat, thereby making the determination more or less vague. However, technical efforts are actively being made to develop semiconductor gas sensors for determining freshness. As shown in FIGS. 6 to 9, a semiconductor gas sensor is generally composed of an insulating substrate 1, a pair of electrodes 12, and a gas sensitive layer 13. Each of the semiconductor gas sensors in FIGS. 6 and 7 is configured by placing the pair of electrodes 12 on the insulating substrate 1 and forming the gas sensitive layer 13 on the insulating substrate 1 and also on the pair of electrodes 12. Each of the semiconductor gas sensors in FIGS. 8 and 9 is configured by forming the gas sensitive layer 13 on the insulating substrate 1 and placing the pair of electrodes 12 on the gas sensitive layer 13. In FIGS. 6 to 9, the pair of electrodes 12 indicates thick film electrodes.
A semiconductor gas sensor has recently been developed to sense trimethylamine, a malodorous component emitted from raw fish, for determining freshness for the fish. Oxide semiconductors based on titanium dioxide are commonly used as a sensitive material for the purpose described above, wherein addition of metal catalyst components to the titanium dioxide improves the sensitivity of the sensor. In this case, the sensitivity of the sensor depends on action and dispersed state of the catalyst and the thickness of the sensitive film, and the type of catalyst components and the amount of their addition play an important role in improving catalytic action described above. In addition to use of titanium dioxide as a sensitive material as mentioned before, indium oxide supplemented with magnesium is under study as sensitive material of a gas sensor for trimethylamine, wherein atomic control by addition of 5 mol % of magnesium oxide to indium oxide reduces electron density, thereby increasing resistance of the sensor in air to make it more sensitive. However, the study of sensitivity of this type of gas sensor for trimethylamine is still at an early stage to apply, and what is worse, too power-consuming to make its mass production feasible.
For vegetables, which emit sulfides gas (mercaptans) unlike trimethylamine coming out from raw fish, a sensor with an excellent sensitivity to determine freshness for vegetables has already been developed. Japanese Patent No. 2875174 describes a method of manufacturing a sensor to determine freshness for vegetables, comprising the steps of adding a given amount of palladium powders to tin oxide powders, mixing them, and then crushing them; calcining the crushed powder mix of tin oxide and palladium at a given temperature for a given time, and then mixing it with an organic material to make paste; coating the paste onto the electrode surface on the substrate to form a sensitive film; and drying the coating and then sintering it at a given temperature for a given time, thereafter connecting a lead wire to the electrode surface.
It has become clarified that a trace amount of ethylene, ethanol or aldehydes is emitted even from fresh vegetable or fruit, while mercaptans are emitted as vegetable or fruit begins to rot and amines, such as ammonia, are emitted as fruit begins to rot. It has also become clarified that sensing a gas, such as ethylene, ethanol or aldehydes is effective for determining freshness for vegetable or fruit, while sensing mercaptans or amines, such as ammonia, is suitable for determining putridity for vegetable or fruit. However, for the conventional type of sensors to determine freshness for vegetables described above, the sensitive film of which is formed by mixing tin oxide powders with a given amount of palladium powders, crushing and calcining them, and then mixing them with an organic material, resulting in production of paste, which is subsequently coated onto the electrode surface on the substrate, dried and sintered, it is difficult to detect one ppm level of ethylene, ethanol or aldehydes, which is necessary to determine freshness for vegetables, and also is difficult to detect one ppm level of mercaptans or amines which is necessary to determine the putridity for vegetables and fruits.
To describe it more specifically, in conventional examples illustrated in FIGS. 6 and 7, the gas sensitive layer 13 jammed between the thick electrodes 12, 12 is obviously formed of a correspondingly thick film due to the thickness of the electrodes in order to achieve a good electric joint. As a result, the problem that the sensor has a low sensitivity arises.
Further, in conventional examples illustrated in FIGS. 8 and 9, since the gas sensitive layer 13 is formed first on the substrate 1, followed by formation of the electrodes 12 over the gas sensitive layer 13, another problem arises that formation of the electrodes 12 may cause contamination of the gas sensitive layer 13 with some impurity.
The present invention is to eliminate the problems described above, and to provide gas sensors highly sensitive to such gas as ethylene, ethanol, aldehydes, mercaptans or amines, and the method of manufacturing them with a good reproducibility.
One aspect of the present invention is a gas sensor characterized in that said gas sensor comprises at least:
an insulating substrate;
a pair of thin film electrodes which are spaced apart at a given interval and provided on said insulating substrate;
a thin film gas sensitive layer which is provided on both said substrate and said thin film electrodes, said gas sensitive layer containing a given material as main ingredient; and
a pair of thick film electrodes which is correspondingly positioned over said pair of thin film electrodes and provided on said thin film gas sensitive layer,
wherein said thin film electrodes and said thick film electrodes are formed so as to sandwich portions of said thin film gas sensitive layer between said two types of electrodes.
Another aspect of the present invention is a gas sensor characterized in that said gas sensor comprises at least:
an insulating substrate;
a pair of thin film electrodes which are spaced apart at a given interval and provided on said insulating substrate;
a thin film gas sensitive layer which is provided on both on said substrate and said thin film electrodes, said sensitive layer containing a given material as main ingredient; and
a pair of thick film electrodes which is correspondingly positioned over said pair of thin film electrodes and provided in contact with said thin film electrodes.
As mentioned before, the gas sensor according to the present invention has a thinner film gas sensitive layer between the thin film electrodes and thus a better electric joint between the thin film gas sensitive layer and the thin film electrodes, compared with the conventional thick film type of gas sensor. This results in higher sensitivity, better stability and longer life of the gas sensor.
Problems associated with the conventional method, that is, adverse effects of the electrodes on the gas sensitive layer 3 in the manufacturing process, such as contamination, cannot arise in the method according to the present invention. It is because the thin film electrodes 2 have been formed before the thin film gas sensitive layer 3 is formed, raising no problems with the thin film electrodes 2. Although the thick film electrodes 4 are formed after the thin film gas sensitive layer 3, a portion of the gas sensitive layer 3 which is directly related to gas detection, namely, the portion P of the gas sensitive layer 3 placed between a pair of thin film electrodes 2 is considerably distant from the thick film electrodes 4, so that it is hardly affected adversely by formation of the thick film electrodes 4.
Even when a pair of thick film electrodes, corresponding to the pair of thin film electrodes, is provided on the thin film gas sensitive layer, as in examples shown in FIGS. 1 and 2, the thin film electrodes and thick film electrodes are configured so as to sandwich the thin film gas sensitive layer. Since electric current flows toward thinner electrodes, there exists virtually direct electric connections between the thin film electrodes and the thick film electrodes, indicating a good electric joint between the thin film electrodes and the thick film electrodes.
The method of manufacturing a gas sensor according to the present invention is characterized by providing a process of coating and then firing an organic solution containing a metal tin salt, an organic compound capable of coordinating to at least a tin, and an activator in order to form a thin film gas sensitive layer. Another method of manufacturing a gas sensor according to the present invention is characterized by providing a process of coating and then firing a paste consisting of a metal tin soap, an activator, and an organic solution containing a viscosity controller in order to form a thin film gas sensitive layer.
In the invention, an organic solution containing a metal tin salt, an organic compound capable of coordinating to at least a tin, and an activator is employed in order to form a thin film gas sensitive layer. Generally, a metal tin salt tends to be hygroscopic and/or hydrolyzable, and so it makes it difficult to produce gas sensitive layers of the same thickness or composition reproducibly. Therefore, an organic compound capable of coordinating to the tin is added to form coordination compounds by partial substitution, thereby achieving stabilization of the metal tin salt. In addition, a paste consisting of a metal tin soap, at least an activator, and an organic solution containing a viscosity controller can also be employed. A metal tin soap forms a micell with some organic solvents, resulting in increased viscosity of the solution so that it may be pasty. A viscosity controller helps to adjust the solution viscosity after it is dissolved in the organic solvent. An activator described above is one of metal salts other than tin salts. Addition of such a metal salt elevates the sensitivity and/or selectivity of the gas sensitive layer to a gas to be detected. A composition for producing a gas sensitive layer comprising an organic solution which contains a metal tin salt, a metal palladium salt, an organic compound capable of coordinating to a tin, and an activator, or a paste which consists of a metal tin soap, at least an activator, and an organic solution containing a viscosity controller is coated and then fired, resulting in reproducible production of gas sensitive layers where the activator is dispersed uniformly. As a result, more highly sensitive gas sensors, capable of detecting down to 1 ppm of different gases (ethylene, ethanol, aldehydes, mercaptans, and amines), can be manufactured, compared to the gas sensor to determine freshness for vegetables, produced in the process comprising the steps of adding a given amount of palladium powders to tin oxide powders, mixing them, and then crushing them; calcining the crushed powder mix of tin oxide and palladium at a given temperature for a given time, and then mixing it with an organic material to make paste; coating the paste onto the electrode surface on the substrate to form a sensitive film; and drying the coating and then sintering it.